This invention relates to improvements in a composite wood beam assembly which is used as a joist to support, for example, building roof structures and the like. An example of such a composite wood beam is disclosed in my U.S. Pat. No. 4,501,102 issued Feb. 26, 1985. In that patent, the described beam is formed of wood chords connected together by wood web members and metal web members which provide a strong, durable, beam assembly useful as a joist.
Beams useful as joists have been made of assemblies of wood parts, that is, wood chords and webs. The capability of such wood beam assemblies to support loads, such as directly applied roof loads, tensile and compressive loads, shear stresses and the like, are generally known and can be calculated by those skilled in the art. The maximum load and stress carrying ability of a beam is generally related to the size of the beam, that is, its length and its cross-sectional size, as well as the particular material of which the beam is fabricated. As a general rule, larger loads require larger cross-section beams or more beams of lesser cross-sectional sizes.
The beam of this invention incorporates changes in several of the beam elements which coacting together, substantially increase the load carrying capacity of the beam without increasing the cross-sectional size of the beam or necessitating increasing the quality of the wood used in its construction. One such change is directed towards the wire staples which have been utilized in the past for fastening the wood members together. Such staples have been made of relatively stiff wire that is bent into U shapes. Typically the staples have been connected together into rows or strips which are inserted within conventional stapling guns for application of the staples. Such staples have been used either in place of or along with nails.
Such staples, as well as the commonly used nails, have a tendency to loosen relative to the wood within which they are embedded, particularly due to the expansion, contraction and bending which normally takes place in wood structures. Thus, while stapling is a fast and economical way of making mechanically fastened joints or connections between wood members, a relatively large number of staples are needed to meet particular strength requirements and even then, loosening of the staples can occur over a period of time.
The typical long, U-shaped staples have legs that are considerably longer than their bases or bights. Thus, there is a tendency for the long, relatively wire staple legs to wander off their driven courses during the staple gun insertion which results in their breaking through the sides or edges of the wood members. The exposure of some of the staples through the surfaces of the wood members occurs frequently. The staple of this present invention obviates this problem.
Further, the wood members which are used for beams are typically made of uniform cross-section lumber. Therefore, they are generally of uniform strength along their lengths. Consequently, in order to handle any localized larger forces or loads at specific areas in a joist as compared to areas handling local forces, the wood members must be of a sufficient size throughout their lengths to handle the larger anticipated loads. For example, if a particular anticipated load requires a 2.times.6 inch beam because of heavy shear stresses at opposite ends of the joist, then the entire beam must be made of that particular size even though other areas of the joist do not require that large a size. Thus, this invention relates to a means for locally increasing the end shear stress bearing capacity of a particular size beam so as to permit the use of smaller cross-sectional size beams or less beams for a particular building construction.